The present invention relates to field-effect cathodes (also called field emission arrays, i.e. FEAs). These cathodes have been already used in certain types of experimental high-power electron tubes, such as relativistic magnetrons, vircators, etc., but also in new tubes of more conventional types, such as travelling wave tubes for applications in radar or in telecommunications.
In this second case, the cathode is formed from at least one array of tips, comprising a substrate covered by a dielectric layer with cavities, each cavity receiving a protruding emissive tip, and a grid placed on the surface of the dielectric layer at least partially surrounds the cavities.
To extract electrons from the tips, a potential difference is applied between the grid and the tips. The electron emission may be modulated in terms of density by modulating the voltage applied to the grid.
From an electrical standpoint, the grid and the substrate/tips assembly, which are separated by the dielectric layer, are equivalent to a capacitance of the order of 10 to 100 pF/mm2 and the corresponding conductance is of the order of a few tens of mS/mm2 at around 10 GHz.
Typically, if about 80 V is applied between the grid and the substrate/tips assembly, it is possible to extract a current of 1 xcexcA/tip, the tips having a density of the order of 106 to 107 per square centimetre.
At frequencies from 10 to 100 kHz, the impedance presented by the grid to the modulator which feeds it is essentially real and remains about a few tens of ohms. This allows a modulator of reasonable power to be used.
Developments underway relate to the operation of these microwave field-effect cathodes. The advantage of an electron tube using such a microwave-modulated cathode is that it can be very compact, it can be constructed without the need for a focussing device and it has a high efficiency. It may be expected to obtain tubes whose operating principle will be comparable to that of IOTs (Inductive Output Tubes), but which operate at much higher frequencies.
However, if the grid is microwave-modulated, the impedance presented by the grid to the modulator which feeds it becomes very low because the capacitor has a very low reactance (0.1 to 1 xcexa9/mm2 at around 10 GHz, for example). This requires a modulator with a bandwidth equivalent to that of conventional tubes, with a very high power, in order to obtain a satisfactory current intensity.
The modulator is connected to the grid via a microwave transmission line, generally a microstrip line. Another reason for requiring the modulator to have a high power is that the modulation signal applied to the gate is reflected at the transition between the transmission line and the grid.
In this regard, FIG. 1a shows, seen from above, a field-effect cathode of known type. The cathode 1 comprises four sector-shaped tip arrays 2, grouped together on the same electrically conducting support 50. Each array comprises a conducting substrate denoted 3, a dielectric layer denoted 4 with cavities 5 in which emissive tips 6 are placed, the dielectric layer being surmounted by a grid 7. Reference may also be made to FIG. 1b. 
The electrical power for each of the arrays 2 is supplied by means of microstrip lines 8 each connecting a tip array 2 to a power modulator M located some distance away. The diagram in FIG. 1a shows one modulator M for each tip array 2, but only one modulator may suffice for all of them. The microstrip lines 8 are long and occupy a much larger area than the tip arrays 2. The modulator M cannot be brought very close to the tip arrays 2, as it is much bulkier than the tip arrays.
In the configuration described and illustrated, the conducting support 50 serves as a conducting plane for the microwave lines 8. The insulation of the microstrip lines is denoted 8.2 and the conducting strip is denoted 8.3.
Each microstrip line 8 is electrically connected to a tip array 2 via a conductor 9 attached on one side to the conducting strip 8.3 and on the other side to the grid 7 of the tip array 2.
The modulators M must generate a high-level microwave signal, especially because, since they are located quite far from the tip arrays 2, they are connected thereto via lines which cause a strong reflection on the grid side and because reflections also occur in the tip arrays 2 owing to the presence of the tips 6.
The further away from the microstrip line 8, penetrating the tip array 2, the weaker the signal and the lower the current density produced by the tips. This results in an inhomogeneous electron beam, prejudicial to proper operation of an electron tube. The modulation signal becomes ineffective beyond 100 micrometres"" propagation into the tip array 2.
The sector shape given to the tip arrays 2 makes it possible, if a width of 50 to 100 micrometres is not exceeded, to improve the homogeneity of the beam. However, the current density is limited as it is not possible to get close to a large number of tip arrays without considerably increasing the area occupied because of the space taken up by the microstrip lines coming from the modulator M.
It is an object of the invention to provide a cathode not having these drawbacks. The present invention provides a microwave-modulable, field-effect cathode, formed from at least one array of emissive tips, which is capable of emitting electrons with a current density much higher than that of the existing field-effect cathodes. This cathode has the advantage of requiring neither a conventional power modulator for controlling electron emission nor a high-level transmission line. Conventional modulators are expensive, greedy in terms of electricity and pose cooling problems. Transmission lines pose problems of differential phase lags in the microwave signal and attenuation problems.
To achieve this, the present invention is a microwave-modulable field-effect cathode, comprising at least one emissive tip array, means for producing a microwave modulation signal and means for conveying the modulation signal to the tip array, characterized in that the means for producing the modulation signal comprise a microwave-controllable semiconductor modulation element which is situated very close to the tip array, the means for conveying the modulation signal to the tip array being a short microline which introduces practically negligible perturbation and achieves impedance matching between the tip array and the semiconductor modulation element.
The microline is especially a line of the microstrip or coplanar type, the conducting strip of which is connected at one of its ends to the tip array and at he other end to the semiconductor modulation element.
The semiconductor modulation element is of the transistor, especially MESFET, type or of the diode type.
To achieve impedance matching, the conducting strip of the microline may be configured divided into two lengths joined together by a capacitor.
The microline may also have a bias function and be connected to a bias voltage source.
At least one element from among the tip array, the semiconductor modulation element and the microline is a discrete component.
At least two elements from among the tip array, the semiconductor modulation element and the microline are attached to the same electrically insulating or semi-insulating support. The two elements may be mounted on one side of the support, the other side of which is coated with a conducting layer which serves as an earth plane.
It is possible to connect the microline to the tip array and/or to the semiconductor modulation element via a wire link.
However, to prevent emission perturbations, it is advantageous to avoid wire links into the tip array. The tip array comprises an electrically insulating or semi-insulating substrate with, on one side, a conducting or semiconducting layer, emissive tips in electrical contact with the conducting or semiconducting layer, a dielectric layer provided with cavities, each housing one of the tips, the dielectric layer being surmounted by a conducting grid which at least partially surrounds the cavities. Passing through the substrate is at least one plated-through hole which is used to electrically connect the tips to the other side of the substrate. The plated-through hole may be extended by a contact which is attached to a suitable conducting contact pad on the support.
Passing through the substrate and the dielectric layer may also be at least one plated-through hole which is used to electrically connect the gate to the other side of the substrate. Wire links associated with the tips and/or the gate can therefore be eliminated.
To eliminate one or more wire links into the semiconductor modulation element, it is possible to use a component compatible with a flip-chip technique.
The microline can be easily produced in a form integrated into the electrically insulating or semi-insulating support, even if the tip array and/or the semiconductor modulation element are discrete components.
To obtain a compact and relatively inexpensive tip-effect cathode, it is beneficial for the tip array, the microline and the semiconductor modulation element to be integrated on the same semiconductor substrate. Preferably, the semiconductor employed is a semi-insulator such as silicon carbide.
The microline may therefore have a strip which is extended on one side to form a grid for the tip array and on the other side to form a contact for the semiconductor modulation element.